The way optical networks are used is undergoing significant change, driven in part by the huge growth of traffic such as multimedia services and by the increased uncertainty in predicting the sources of this traffic due to the ever changing models of content providers over the Internet. Sophisticated modulation schemes for higher bandwidth 100 Gb/s services and beyond are known and come into commercial use in optical networks of large and increasing link and node numbers. A bottleneck to widespread deployment of such schemes is the “fixed” wavelength grid approach specified by the International Telecommunication Union (ITU), in which the relevant optical spectrum range in the C-band is divided into fixed-sized spectrum slots. Such conventional “fixed grid” WDM (wavelength divisional multiplexed) networks work on the concept of a fixed spectrum grid typically with a spacing of typically 50 GHz between channels with 80 to 100 of these channels per fiber. In these networks, an individual signal serving a demand between two nodes in the network has to keep within one of these channels or slots defined by guard bands, as otherwise the signal becomes notched and degraded by the wavelength filters when being split from its neighboring signal. As a result of this restriction, advanced modulation formats allowing up to 100 Gbit/s per 50 GHz channel commercially and up to 200 Gbit/s experimentally, have not to now been usefully deployed in a widespread manner. This is because the spectral widths of such signals are wider than can be accommodated within the 50 GHz fixed grid spacing, so the potential of additional increases in transmission speed cannot be realized.
As used herein, a “slot”, “wavelength” or “channel” is defined as a wavelength or a spectrum of wavelengths associated with a certain signal size. A “carrier” carries a “signal” or “demand” in the known fashion. As is also known, a connection between nodes is made by assigning spectral (i.e. wavelength) slots on the optical links comprising the path between source and destination.
A response to the problems posed by the decade-old ITU fixed grid approach is the flexible grid or “flexgrid”, which facilitates a developing optical networking paradigm known as EON (elastic optical networking). The EON technologies allow for radically different network design and operation methodologies that can increase the amount of traffic the network can carry compared to conventional WDM networks, but need different processes to make them operate effectively to get the most out of such networks. In the flexgrid approach, the optical spectrum can be divided up flexibly in dependence on requirements, and elastic optical paths (i.e. paths with variable bit rates) can be generated. This allows for operational and functional flexibility in use of both the optical spectrum and transceivers, previously unavailable in fixed grid implementations. In a flexgrid, the spectrum grid is divided into much finer slot widths, typically 12.5 GHz or less, compared to the 50 GHz in the fixed grid approach. Significantly, adjacent channels can be joined together to form arbitrary sized slots to carry signals of a variety of widths, allowing for signals ranging in size from an individual channel to that occupying the entire optical spectrum to be carried.
Representations of signals carried according to the fixed and flexible grid approaches are depicted in graphs shown in FIG. 1, in which graph (a) is a depiction of the fixed grid approach, in which guard bands (2) partition adjoining optical channels (4) occupied by demands or wavelengths at a particular bit rate. The guard bands serve to separate a demand from other demands going to other destinations and to protect the main signal as it passes through filters in the network so as to reduce the effects of passing through the optical nodes. Graph (b) illustrates the flexgrid approach used in an EON network, in which the demands (here shown to be of various spectral widths) are not constrained within a slot of pre-defined spectral size. As illustrated by demand (4b) in graph (b) of FIG. 1, a high bitrate demand with a spectral width exceeding fixed grid slot sizes can be accommodated. A “superchannel” (6) (depicted in graph (c) of FIG. 1) for carrying demands which are too large to be handled by a single optical channel, can similarly be accommodated in an EON network. A superchannel comprises a grouping of multiple channels and is handled as a single entity, traversing the network for demultiplexing at the receiver end. Specifically, they can be produced by a bandwidth variable transponder (BVT) enabling a number of carriers. The carriers are aggregated together at the transceiver to produce an optical signal of a size which depends on the level of traffic carried by the signal. So if more traffic needs to be carried by the BVT, additional carriers can be added and conversely, if traffic levels decrease, carriers can be disabled. A general description of the use of BVTs in an EON network can be found in “Elastic Optical Networking: A New Dawn for the Optical Layer?” by O. Gerstel, M. Jinno, A. Lord, SJB Yoo (IEEE Communications Magazine, February 2012). The operational flexibility of superchannels can usefully cope with growth in traffic levels in a network over time, and significant spectral savings can be gained over the fixed grid approach. BVTs can be used in both fixed grid and flexgrid systems, although they are deployed to greater effect in flexgrid networks owing to the capacity of the latter to accommodate the greater spectral widths of superchannels.
In a conventional WDM network, a node receives signals from one or more other nodes in the network, typically via intermediate nodes. Currently, all these signals meant for the same destination node are each considered in isolation, and known routing techniques such as routing and spectrum assignment (RSA) algorithms allocate a route and spectrum to a signal regardless of its source and destination. As a result, signals originating from different nodes but destined for the same node can be allocated spectrum or blocks of spectrum potentially anywhere across the entire C-Band even on the same link. Even between the same source/destination node pair, multiple signals (caused by the need to carry increasing levels of traffic across the network) are likely to use different parts of the spectrum and possibly different routes across the network. Conversely, signals destined for different nodes can be placed on neighboring or contiguous sections on the optical spectrum along a link or route. Because no filter can be entirely precise, guard bands between demands are essential to provide a kind of “padding” between signals to be split within the spectrum for transmission to different destinations, and also to allow an operational tolerance during the splitting process. Guard bands occupy part of the spectrum alongside the demands they separate, and so as might be expected, the spectral requirements for guard bands increase as the number of discrete signals or number of nodes in the network increases.
It would be desirable for more of the precious spectrum resource to be given to carrying signals or demands instead of being occupied by components such as guard bands. By making better optimized use of existing optical fiber assets by serving the maximum number of customer demands possible on any given fiber link or network, the need to expensively and disruptively install additional fiber and transceivers can be prevented or at least advantageously delayed.
The applicants' co-pending applications WO2014174234 and GB1317987.4 describe a method of routing signals in a WDM network which is based on the level of “network entropy” of an identified route (representing the extent to which a spectrum comprises non-contiguous used or unused sections within it), which are incorporated by reference herein. U.S. Pat. No. 5,596,722 also considers network routing entropy in the context of satellite communication system which is not based on WDM. None of these documents address the issue of saving spectrum for more productive allocation to demands or signals in preference to other components or requirements such as guard bands.